Pianotuna, I don't know about your trailer but you might think about a rack that attaches to the sidewall of the trailer rather than the roof. This way you may be able to use a little larger supports. I have attached this link that shows a roof rack for horse trailers to haul hay. Only reason is to show that they are attached to the sidewall and don't have to drill into the roof. You don't mention if the trailer will be moved but if it is and the panels are up in the air you will need a little more sturdy supports to handle wind whipping up under. Another option is the attach the panels to the top side of the trailer and lift them up from the bottom when you get parked to whatever angle you want them. Both these options require no drilling into your roof.
clicky